If the tech sector is to increase the number of women in its workforce, schools must develop robust, mandatory computer science programs in the K-12 education stage, according to a prominent advocate for women in tech.

"You make it an option, the girl is not going to take it. You have to make it mandatory and start it at a young age," says Ashley Gavin, curriculum director at Girls Who Code, a nonprofit working to expose more girls to computer science at a young age that has drawn support from leading tech firms such as Google, Microsoft and Intel.

"It's important to start early because, most of the fields that people go into, they have exposure before they get to college. We all study English before we get to college, we all study history and ... social studies before we get to college," Gavin says. "No one has any idea what computer science is. By the time you get to college, you develop fear of things you don't know. Therefore early exposure is really important."

Gavin, speaking at an event focused on increasing the number of women pursuing education in the fields of science, technology, engineering and math, claims that, of the job openings in those STEM fields, some 70 percent will be pegged to computer science by 2020.

Few women in tech, more jobs left unfilled

Advocates of expanding STEM education often talk about the "leaky pipeline" - the fact that many students who initially start toward a degree in one of those fields change course and pursue a different discipline.

Girls Who Code estimates that 30 percent of students with early exposure to computer science stay in the field. At that drop-off rate, for women to fill half of the computer specialist job openings the Department of Labor is projecting in 2020, 4.6 million girls will need some K-12 computer science instruction. Gavin's nonprofit alone aims to reach 1 million girls by 2020.

Other industry data paints a starker picture. Women comprise 48 percent of the total workforce but hold just 23 percent of the STEM jobs, according to the National Math and Science Initiative. In 1991, women received 29.6 percent of the bachelor's degrees awarded in computer science; in 2010, women took just 18.2 percent of those degrees.

Gavin concedes that it's bigger than just the question of participation among women. "Yes, there are very few women pursuing computer science, but there are also very few people pursuing computer science," she says. "It's not just a women problem, it's a people problem. It's a real people problem."

Promote STEM by highlighting tech job satisfaction, salaries

Students can name any number of objections to pursuing computer science, so there's no single approach to sell skeptical kids on working toward a career in the field.

The fact that STEM workers earn higher wages can be a good start. On average, employees in STEM jobs earn 26 percent more than workers in other fields, according the Department of Commerce. "For some kids, it's money," Gavin says. "Especially for kids in the underprivileged communities, money is a huge motivator."

She also cites studies suggesting that software engineers are among the workers most satisfied with their careers. Other students might be inspired by the potential to harness technology to effect positive change in the world, for instance, and might imagine themselves developing apps that improve healthcare or promote energy efficiency.

"But more than anything, even though you say this to them, and even though they recognize how cool it is," Gavin says, "You can show them video games, robots, mobile apps, whatever [and they say, 'I get all that. It's not for me.'"

Adds Hannah Valentine, senior associate dean at Stanford University: "At the core of it is a sense of not belonging."

That argues for the importance of an experiential component to STEM exposure. Often, when students step inside the headquarters of a tech firm and get to interact with smart, motivated workers in a fun environment, that's when it clicks, Gavin says.

To that end, she urges educators to reach out to local businesses to arrange a visit for their students. "These tech companies are way more open to bringing kids in than you think you are," Gavin says. "If you can get your kids in on a field trip to any tech company - it doesn't even have to be the sexiest one, it doesn't have to be Google."